1. Field of the Invention
The present invention relates to an inkjet marking module comprising an inkjet marking device being adapted to jet droplets of an inkjet marking material to form an image on recording substrate; and an evaporation device arranged for evaporating a solvent in a gaseous medium, the evaporation device being arranged to control the vapor pressure of a solvent vapor in a gaseous medium present in the printing module within a predetermined range in order to prevent drying of the inkjet marking material in or on the inkjet marking device, hence preventing deterioration of the jetting properties of the inkjet marking device. The present invention also relates to a printing system comprising such an inkjet marking module and a method for controlling the vapor pressure of a solvent vapor in a gaseous medium in such an inkjet marking module.
2. Description of Background Art
Deterioration of jetting properties of an inkjet printing device (inkjet head of printhead) is a known problem in the background art. It is also known that nozzle clogging of inkjet heads with dried ink residues and depositions of dried ink residues on a nozzle plate near the nozzles cause jetting properties to deteriorate. Ink residues causing nozzle clogging and said depositions are generated due to evaporation of solvents from the ink composition present in the nozzles and on the nozzle plate. To solve the problem of evaporation of solvents from the ink, the background art suggests providing a micro-climate in the nozzle region of the inkjet printing devices. The micro-climate is provided by supplying air enriched with solvent vapors to said nozzle region (see for example U.S. Pat. No. 5,929,877, U.S. Pat. No. 7,604,322 and U.S. Application Publication No. 2007/0285456). The micro-climate prevents evaporation of the solvents present in the ink in the nozzle region and hence ink residues do not dry out. Clogging is effectively reduced and depositions of ink residues on the nozzle plate can be easily wiped off.
In the case of water based inks (solvent is water) the micro-climate is provided by supplying air enriched with water vapor (see U.S. Application No. 2007/0285456). In this published U.S. patent application, it is also disclosed that for controlling the humidity in the nozzle region, a humidity detection portion (i.e. one or more humidity sensors), is arranged for periodically measuring the relative humidity (RH) in the nozzle region when an ink carriage comprising an ink cartridge passes said humidity detection portion in a scanning printing process.
In U.S. Application Publication No. 2011/0115863, a humidifying unit that generates humidified gas and the use thereof in an apparatus also including a drying unit and a recording unit are disclosed. It is also disclosed that the humidifying performance is feed-back controlled by a control unit on the basis of a humidity sensor, so that the humidified gas at an appropriate humidity can be generated.
A disadvantage of the humidifying methods and devices as disclosed in U.S. Application Publication No. 2007/0285456 and U.S. Application Publication No. 2011/0115863 is that humidity sensors are not capable of preventing super-saturation or even condensation if the RH to be controlled (setpoint of the humidity sensor) deviates from 100% (e.g. saturation) by less than the accuracy of the humidity sensor. For example, at a humidity sensor accuracy of ±3 RH % (common for humidity sensors) and a setpoint of 98% RH, the controlled RH may vary within the range of between 95% and 101%, based on the measurement accuracy alone (thus not including oscillation around the setpoint). Therefore, adequate control of a RH near the saturation point while preventing supersaturation and/or condensation is not possible.
It is another disadvantage of the humidifying methods and devices as disclosed in U.S. Application Publication No. 2007/0285456 and U.S. Application Publication No. 2011/0115863 that directly controlling the air humidity based on humidity sensing is too slow in a high speed printing process, such that the air humidity in the nozzle region is not properly controlled, which may lead to drying out of ink residues and hence to nozzle clogging or to condensation, both deteriorating the jet properties.
It is yet another disadvantage of the humidifying method and device as disclosed in U.S. Application Publication No. 2007/0285456 that the arrangement of the humidity sensor is such that the relative humidity in a region near the nozzles is only periodically measured, when the ink carriage comprising the ink cartridge passes the humidity sensor. Continuous and accurate humidity control is therefore not possible.